One of the most important scientific advances in the post-genomic age has been the elucidation of gene and environment interactions. The existence of these interactions has been well-established, but only within the past decade could the details of these interactions be examined at a molecular level. This application proposes to examine the effects of day length on estrogen receptor (ER) expression, aggression, and mating behavior in beach mice. First, the effects of long days and short days on ERalpha and ERbeta protein and mRNA will be examined. Second, the functional consequences of ER regulation will be examined with hormone manipulation experiments that will test the effects of ERalpha and ERbeta on aggression and mating behavior in long days and short days. Finally, this research will examine how ER positive cells respond in different social contexts. Importantly, the behavioral responses of the beach mouse resemble those of humans in that behavioral changes are observed across seasons, but reproductive activity is not suppressed by short days. There is a growing body of clinical research which indicates that estrogenic function has important effects on aggressive behavior and mental disorders. The proposed research should provide a novel and important framework to understand how the environment interacts with ER function to modulate behavior.